Pulsus paradoxus is a medical term referring to a quantifiable, exaggerated decrease in arterial blood pressure during inspiration. In normal patients, this decrease is in the range of about 2–5 mm Hg; whereas, in a patient suffering from certain medical conditions, pulsus paradoxus can reach 10 mm Hg or higher. Pulsus paradoxus has been noted in a variety of medical conditions including, but not limited to, upper airway obstruction, bronchial asthma, tricuspid atresia, mitral stenosis, conditions of decreased left ventricular compliance, croup, tension pneumothorax, pericardial tamponade, pulmonary embolism, hypovolemic shock, and sleep apnea.
Asthma and Related Diseases
Asthma accounts for almost two million Emergency Department admissions annually in the U.S., and it is estimated that 29 million people will be diagnosed with asthma in the next two decades. Hospitalizations for childhood asthma have increased 3% to 5% annually, and mortality from asthma has increased 10% annually since 1977. Other forms of lung disease, including but not limited to chronic obstructive pulmonary disease (“COPD”) and emphysema, place a heavy burden on patients and on the system of medical care. Early recognition and accurate assessment of the severity of airway obstruction and of the response to therapy are fundamental to the improvement of health for persons with these disorders.
Common measures used currently to assess the severity of asthma are clinical assessment, arterial blood gas analysis, spirometry, and pulse oximetry; however, all are subject to certain shortcomings. Clinical assessment scores, for example, exhibit marked interobserver variability and have been incompletely validated. Arterial blood gas analysis is an invasive and painful technique and is often complicated by therapeutic administration of O2 and β-adrenergic drugs and is therefore unreliable as an indicator of asthma severity. Tests of forced expiratory flow, as in spirometry, are effort dependent, typically cannot be used with children, and can actually exacerbate the underlying disease process. In part because physicians' ability to accurately assess pulmonary signs can be unreliable, numerous clinical scoring systems and management guidelines have been established for diseases such as asthma.
Despite the publication of the National Heart Lung and Blood Institute (“NHLBI”) Guidelines for Emergency Department Asthma Management in 1991, the asthma mortality rate in children ages 5–17 nearly tripled between 1980 and 1996. Many experts are at a loss to explain the rising mortality of asthmatic patients in view of the improving quality of acute pharmacological management of asthma and the enhanced sophistication of emergency physicians and pre-hospital care systems. One explanation lies in the observation that there has been little change in how the acute asthmatic patient is evaluated. A recent development in assessing acute asthma has been the use of pulse oximetry (SPO2) which measures the degree of oxygen saturation of hemoglobin non-invasively and empirically. Despite the ubiquitous availability of pulse oximetry, (-adrenergic drugs, used widely, can result in ventilation-perfusion mismatch, leading to a fall in SPO2 even though the patient is improving. Finally, changes in SPO2 reflect atelectasis and intrapulmonary shunting and do not directly provide information regarding airflow obstruction and ventilation. An easily recognized, sensitive and objective parameter, by which practitioners of all levels of expertise could quickly recognize an asthma exacerbation, would help in more accurate diagnoses.
Easily measured, objective and accurate indices of severity for acute exacerbation of bronchiolitis, croup, emphysema and COPD are also not available. Additionally, medical emergencies such as cardiac tamponade, hypovolemia, and pulmonary embolism are difficult to diagnose and/or to quantify in severity. Finally, the response of these disorders to treatment is likewise difficult to objectively measure.
Traditional Methods of Pulsus Paradoxus Measurement
Although measurement of pulsus paradoxus is recommended by numerous authoritative medical practice guidelines (for example, the previously cited NHLBI Guidelines for Emergency Department Asthma Management), pulsus paradoxus is rarely recorded in clinical practice. Resistance by physicians to the application of pulsus paradoxus for the objective assessment of disease severity, and asthma in particular, is largely due to the difficulty in measuring pulsus paradoxus in a rapidly breathing patient by currently employed methods.
One conventional method of measuring pulsus paradoxus is through the use of intra-arterial catheters. Although intra-arterial catheters can often give reliable measurements of pulsus paradoxus, placement of these catheters is painful and associated with significant risk. Furthermore, placement of intra-arterial catheters should only be done by highly trained medical personnel using sophisticated monitoring equipment, preferably in a hospital setting. Consequently, this method is not favored for general use.
Another method for measuring pulsus paradoxus is with the use of a sphygmomanometer, commonly referred to as a blood pressure cuff. This technique involves inflating the sphygmomanometer to above systolic pressure and slowly deflating it. As the elevated systolic pressure occurring during expiration is approached, heart sounds will be heard intermittently (during expiration only), and as the lower systolic pressure occurring during inspiration is approached, heart sounds will be heard continuously. The difference between these points at which heart sounds are heard, first intermittently and then continuously, is a patient's pulsus paradoxus. However, this traditional method of measurement is difficult in the clinical setting in which noise, rapid respiratory rates and patient movement are the norm, and provides measurement of pulsus paradoxus at only a single point in time. Moreover, this process is ergonomically very difficult to perform and multiple operator efforts are typically required. As a result, the method is often inaccurate and unreliable. Manually derived pulsus paradoxus also correlates poorly with pulsus paradoxus calculated from intra-arterial pressure and, despite the NHLBI's recommendations, actual measurement of pulsus paradoxus is rare.
A third method of measuring pulsus paradoxus has been developed through photoplethysmographic techniques. In the field of photoplethysmography, pulses of light having different wavelengths are transmitted through or reflected by a patient's tissue to non-invasively determine various blood analyte values. More particularly, a photoplethysmographic device known as a pulse oximeter is employed to determine pulse rates and blood oxygen levels. Pulse oximeters typically include a probe that is attached to a patient's appendage (e.g., finger, ear lobe or nasal septum). The probe directs light signal pulses generated by a plurality of emitters through the appendage, wherein portions of the light signals are absorbed by the tissue. The intensity of light transmitted by the tissue is monitored by one or more detectors which output signals indicative of the light absorbency characteristics of the tissue. Because the blood analytes of interest each differentially absorb more light at one wavelength than at other wavelengths, the ratio of detector output signals can be used to compute the blood analyte concentrations.
By way of primary example, it is known that oxyhemoglobin (O2Hb) absorbs light more readily in the infrared region than in the red region, whereas reduced hemoglobin (RHb), or deoxyhemoglobin, more readily absorbs light in the red region than in the infrared region. As such, oxygenated blood with a high concentration of oxyhemoglobin and a low concentration of reduced hemoglobin will tend to have a high ratio of optical transmissivity in the red region to optical transmissivity in the infrared region. The relative transmissivity of blood at red and infrared center wavelengths can be employed as a measure of blood oxygen saturation (SpO2).
It is also recognized that concentrations of other related blood constituents (e.g., carboxyhemoglobin (COHb) and methemoglobin (MetHb)) can be measured with a similar approach since such analytes also have unique light absorbency characteristics at different corresponding wavelengths. The determination of such additional constituents can serve to enhance the measurement of blood oxygen saturation.
As will be appreciated by one skilled in the art, the detector output signal in pulse oximeters contains non-pulsatile and pulsatile components. The non-pulsatile component is influenced by the absorbency of tissue, venous blood, capillary blood, non-pulsatile arterial blood, the intensity of the light signals, ambient environmental light, and the sensitivity of the detector. The pulsatile component reflects the expansion of the arteriolar bed with arterial blood, and the varying amplitude of this pulsatile component depends upon the blood volume change per pulse as a result of arteriolar inflow. As such, the pulsatile component isolates the optical absorption attributable to the arterial blood component of the vascular bed and provides a basis for monitoring changes in the concentration of the noted blood analytes, oxyhemoglobin and deoxyhemoglobin. This feature of all plethysmographic oximeters, isolation of the pulsatile component of the arteriolar vascular bed and the waveform signal so generated, can be used to determine pulsus paradoxus for a particular patient.
Despite the problems inherent in these methods of detecting pulsus paradoxus, the advantages of measuring and monitoring pulsus paradoxus are significant. These measurements provide valuable insight into how troubled the act of breathing is for a given patient, and can help physicians detect, assess and treat numerous respiratory ailments. The NHLBI has recognized the advantages of measuring pulsus paradoxus and has recommended that pulsus paradoxus be measured on all asthmatic patients, despite inherent inaccuracies of the sphygmomanometric technique. Moreover, the NHLBI has advised that any patient with a pulsus paradoxus of 12 mm Hg or greater be hospitalized.